Evolution: Today’s crocodiles have an evolutionary history of over 200 million years. They are believed to have changed very little over the last 20 million years. There are 23 living species of crocodiles and their relatives, like caimans and alligators.

Appearance: The Freshwater Crocodile is slender-snouted and considerably smaller in build and overall size compared to its cousin, the Saltwater Crocodile Crocodylus porosus.

Crocodiles are covered in scales known as scutes, which do not overlap each other. Bony plates called osteoderms provide extra protection, and these are embedded in the skin along the crocodile’s back. These plates act as armour for crocodiles.

Salt glands: Freshwater Crocodiles have salt glands in the tongue. These glands, about 20-26 of them, secrete sodium and potassium in concentrations higher than that of the blood. It is unknown why this primarily freshwater species has salt glands, but it may be that the salt glands exist as an important means of excreting excess salt and maintaining internal water balance during the dry season when the crocodiles lie dormant on land. Or, given that the species occasionally inhabits brackish waters, the excess salt can then be excreted though the salt glands.

Staying underwater: They can slow their heartbeats down to just a few beats per hour, allowing them to stay underwater for over an hour.

Photo by Gary Ramage.

Freswater Crocodiles, also known as Johnstone's River Crocodiles, Johnstone's Crocodiles and "freshies" are a species of crocodile endemic to Northern Australia.

At Taronga:

At Taronga Zoo we have two Freshwater Crocodiles (they do not have names). They are in their 20s and came from crocodile farms.

Habitat

These crocodiles are distributed across far north Australia, from the southwest Kimberley region (WA) to the Cape York Peninsula (QLD). Within these regions, they live various freshwater environments, including rivers, creeks, billabongs, lagoons and swamps. Despite the common name, they may also occur in brackish waters up to 24% salinity (seawater is 35% salinity).

The Freshwater Crocodile has been assessed as a Least concern species by the IUCN Red List (retrieved 19.02.16 from http://www.iucnredlist.org/details/46589/0).

Food may be obtained using the “sit and wait” method i.e. they wait for small animals to come within close range before snapping them up in a sideways motion. Larger prey may be ambushed.

To aid digestion they may ingest small pebbles which help to grind up food in the gizzard.

Breeding

These crocodiles reach sexual maturity at 15 years of age. They breed at the start of the dry season (June) with egg laying taking place about 6 weeks later.

The incubation period for these eggs is 65-90 days, to where 4-24 eggs will have been laid (the average is 12).

Unlike saltwater crocs, the females do not guard the nest but will come back to excavate it once the eggs have hatched, responding to calls of the young. Once uncovered, the female carries them to the water and will guard them aggressively.

The sex of crocodiles is not determined by their genetics, but rather by the temperature of the nest in which the eggs are incubated. Generally, an average temperature of 32 degrees Celsius will produce males, and temperatures slightly higher or lower than this will produce females.

Behaviour

Freshwater Crocodiles are largely nocturnal, meaning they are mainly active at night. They are mostly solitary animals, but may congregate in groups.

A large male will often dominate over a congregation and assert dominance by biting subordinate males.

Threats

The main threats to crocodiles are habitat destruction and the poisonous Cane Toad (which the crocodiles eat with fatal consequences).

We Act for the Wild

Our freshwater crocodiles help to educate visitors about our native wildlife and what people can do to help conserve species.

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